I know it is not a Plymouth but there is a wealth of knowledge about Mopar here and I would like to tap into it, please.
I am unaware of any North American production Chrysler built vehicle that came with a flat head V8 engine. Can you provide more details about what you are referring to?
Out of long time habit, 8 meant V, I meant straight. Sorry.
Here is some information.....
http://www.imperialclub.com/Repair/Engine/straight8.htm
I think the last year for the straight 8 was 1950 and the big V-8 came out in 1951.
AllPar.com has this list.
https://www.allpar.com/mopar/list.html (https://www.allpar.com/mopar/list.html)
an experimental V8 coded A26 was developed in the 40s
a picture of it, dated October 1945 can be found in Willem L Weertman's excellent book Chrysler Engines 1922-1998 on page 123 SAE publications,
It was of L head design but differed from the Ford in that the exhaust ports are along the top of the block.